This invention relates generally to magnetic tape drives for recording and retrieving data, and, more particularly, to mechanisms for preventing inadvertent writing on or erasing a tape. Magnetic tapes are used in conjunction with digital computers for the storage and retrieval of digital data. Since the data on a tape may be valuable or may be expensive to reproduce, it is customary to provide each tape reel with a "write-protect" feature, whereby data on the tape cannot be erased, or overwritten with new data, without some deliberate act of the person loading the tape reel onto a tape drive.
In the past, this protection feature has been implemented by means of a plastic "write ring," which may be fitted in an annular groove in the hub of the tape reel. The presence or absence of the ring is sensed by a probe that is automatically extended from the tape drive when a tape has been loaded. The probe then immediately retracts, to avoid frictional wear that would otherwise take place if the probe were left in contact with the ring.
This approach to tape protection has at least three significant disadvantages. First, the ring has to be mounted on the rear face of the reel, and is not visible once the reel has been mounted on the tape drive. Hence, a last-minute check of the tape's protection status is not possible, and the risk of inadvertent tape erasure is still significant. Moreover, the ring is not visible when the tape reel is on a table or other surface with its labelled front face up. Another drawback is that the "wire ring" is a separate component that has to be separately stored and is subject to being misplaced or lost. Finally, the requirement for a retractable probe adds to the cost and mechanical complexity of the tape drive mechanism, and increases its power consumption.
It will be apparent from the foregoing that there has been a significant need in the field of magnetic tape drives for a technique that will permit the write-protect status of a tape reel to be determined from the front or label side of the reel, and will also permit the status to be quickly changed from the front side of the reel, without the use of separable parts and without using a retractable probe. The present invention is directed to these ends.